Magnetic field gradient measurement



Feb. 17, 1948. C, H, FAY 2,436,939

MAGNETIC FIELD GRADIENT MEASUREMENT Filed Jan. 11, 1945 Q Gain l Coni'rcl g3 A9 7 Phqrocen r2.7 Non-magnetnc Housing FQJ invenfor: Charles H. Faq /v bq his Arrornzg:

Patented Feb. 17, 1948 MAGNETIC mi) GBADIENT MEASUREMENT Charles li. Fay, Houston. Tex., assigner to Shell Development Company, San Francisco, Calif., a corporation of Delaware Appummn .tammy u, isis, semi n. 512,219 a claim; lei. 11s-iss) l This invention pertains to the art Ofmagnetic' I exploration, and relates more specifically methods and instruments for the measurement of magnetic field gradients. Such measurementsl are useful in geophysical exploration. and are of especial value in detecting or locating bodies or objects whose presence underground causes anomalies or disturbances in thex'nag'netic field. In my copending application Ser. No'. 514,581, filed December- 15, 1943, now Patent No. 2,425,- 180, of which the present application is a continuation in part, I have disclosed a device for measuring magnetic fields. said device comprising a vibrator coil, a light train and photocell system adapted to translate the oscillation of the coil into a voltage varying in sympathy therewith. an amplifier for said voltage provided with manual or automatic gain control means', "said ampliiler driving said coil so thatthe whole system operatesi as a feedback oscillator when the vcoil is properly oriented with respect to the magnetic field component to be measured, the amplifica tion gain required to allow oscillation'being inversely proportional to said magnetic eld component, and thus serving as a measure "of said component when properly calibrated.

While said device is extremely useful for determining the location of underground bodies from surface measurements, such determination is not always possible because of the presence on or near the surface of the ground of other magnetic objects which it may be dilcult or impossible to remove. In such cases, however, and especially when the general areav under which the buried body lies is known, it is possible to locate said body accurately by means of magnetic meas a non-magnetic material, is electrically connected to the surface apparatus, generally indicated at 3, by means of an insulated cable l, comprising a desired number ci electrical conductors, of which urements carried out in boreholes which may' have'already been drilled in the vicinity. or may be drilled for the purposes of said measurements,

for example, such holes as can conveniently and rapidly be drilled at various points by means of a hand or power operated auger.

While the instrument disclosed in said copend` ing application may be readily constructed in a form adapted for use in a borehole, it isl the object of the present invention to provide a meth- 5' 0d and apparatus especially suitable for such purpose` intended to restrict the use of saidv instrument i l-v,to measurements in boreholes.

Nothing in this application, however, is

ing l, of a non-magnetic material.

four- I i, I2, i3 and Il-a're shown for purposes of illustration.

The housing lA contains the gradient-sensitive element of the 'present invention, which comprises coils Il and l1, each having a desired number of turns of wire, said coils being rigidly held together by a rod or staff i9, which is made. as all yparts. forming the structure of hous- The coils il and i'lv are mechanically suspended but not elec trically connected, by means of taut wires or filaments 2l and, from brackets 25 and 21; suitably arranged within the housing. Means such as screw-threaded knobs 29 and 3| are pro`- vided to adjust the tension of the suspensions 2| and 2l, and toimpart, when desired, a twisting moment to said suspensions. Coils I5 and I1 are connected, bynnean's of `conductors 33, 35, 31 and 39, across variable resistances Il and 43 in vthe circuit of cable conductors H and l2. Coil l! isconnected in series opposition to coil Il: and the coils are made, with regard to size, number of turns, turn area, etc., asnearly alike as possible, so that in the ideal case where the coils tem to be made so as to have the coils as co planar as possible, the final angular adjustment can be made by applyingv a small twist tothe system by means of knobs 2-9 and 3i. Inequality in turn -areacan be compensated for by regu lating the ratio of the currents in the two coilsv by suitable adjustmentof the values of the variable shunt resistors Il and I3.

A small mirror y45 is rigidly attached to some An oscillation offthe suspended coil .system about its. vertical axis causes the' mirror to modify the degree -ofillumination oi' the photocell by-the reflected beam, whereby the current flowing in the circuit thereof isl variedin sympathy with said oscillation. This current is first amplified in pre-amplifier 50, and then in a manual or automatic gain control amplifier .at the aasaosa surlace. which amplifier may be provided with an ampliiication gain indicator Il, circuits suitable for use in said amplifier and indicator being fully disclosed in my aforesaid copending application. I'he magnetic field due to this ampliiied current, when passed through coils il and i1 by means of conductors Il and I2, will react with the magnetic field under measurement to develop a torque. Coils Il andv I1 being substantially identical and connectedin series opposition, the resulting net torque will be zero under condition ci a uniform natural ileld. When, however, the natural magnetic ileld'has a sensible vertical gradient oi its horizontal component. and the suspended system is properly oriented, the torques oi the; two coils will not cancel each other, the resulting net torque causing the suspended system tooscillate at its own mechanical resonance frequency. the device functioning as a feedback oscillator. The amplication gain which the `ampliiier 5I must have in order barely to maintain the system in oscillation, whichgain is indicated by the properly calibrated device 53. thus serves to measure the magnetic ileld gradient.

The use ci' the instrument may be extended to measurement oi vertical gradients oi the horizontal component of the Lexternal field smaller than required to give suillcient feedback for oscillation. This is eilected by mounting a small magnet 55 of such 'strength and in such position in the housing i as to provide a gradient slightly larger than that required for oscillation: it is obvious that by turning the instrument about a vertical axis the external gradient can be made to add to or subtract from the internal gradient due to magnet 8l, so that the change in reading o! indicator I3 upon said turning serves to measure said external gradient, it ,being assumed that the twisting adjustment and resistor adjustment previously described herein have been made. as can be done by adjusting until the reading of indicator I3 remains substantially constant when the instrument is rotated in a location known to have a gradient of zero value.

In order that the instrument may be operated in a borehole, the housing l may be provided with a coupling li for rods or tubing, such as shown at Il, said 'coupling and rods being rigid to prevent rotation about a vertical axis, so that housing can be turned about said axis, and its orientation observed. Any desired number of rods may be coupled together to permit lowering to a deaired depth, and the depth can be measured, for example, by marking the rods. In operation measurements are taken at a plurality of depths. At each depth the instrument is rotated until a maximum gradient reading is obtained: this reading and the orientation, that is, the direc tion oi' the gradient vector, are recorded.

The location of the buried magnetic body can often be determined from such data for a single borehole, or at least a more favorable location for a second borehole can be chosen from the data, provided, of course, that the first borehole is sumciently near the object for it to give a measurable magnetic eilect. It is obvious that the directional indication obtained with this instrument is ot great value; in this regard said instrument is Superior for borehole survey to an instrument measuring the vertical gradient of the vertical ileld.

- 4 Another application o! the present invention visinthexneasurenientotcurrentsiupipelines,

`when said ileld has wires, and the like by their magnetic eilects. The magnetic neld due to a current in a pipe line, for example, as observed directly over the pipe line. has a horizontal component, and a vertical gradient of that component. at right angles to the direction of the pipe line, so that the present invention can be used to measure said gradient, from which, and the distance from the instru ment to the pipe line, the current in said line can be computed. Although it has been known to eiect the measurement of pipe line currents by their magnetic eii'ects the instruments used therefor normally atl'ord no means for distinguishing between the ileld due to the current in the pipe line and the field due to accidental magnetization oi' the pipe line itself. However, this latter magnetization has usually a negligible horizontal component in the direction at right angles to the pipe above the pipe, and hence a negligible eil'ect on the measurement of the vertical gradient ot horizontal eld in said direction by means of the device of the present invention.

I claim as my invention:

1. In a system for measuring the gradient of a magnetic eld, a unit comprising two spaced coils and means rigidly pivoting said coils in iixed relationship to each other for oscillation about a common axis. means electrically connecting said coils in series opposition, a light source, photo-electric means. optical means carried by said rigid unit for varying the illumination of the photo-electric element by the light source proportionally to the oscillation of said unit, a circuit comprising variable gain ratio ampliner means energized by the output of said photoeiectric element, means for passing the output current of said circuit through said coils. means for adjusting the relative intensity of the current passing through each coil, and indicator means in said circuit for indicating the ampliilcation gain oi' said ampiiiler means necessary for maintaining said coil unit barely in oscillation by the torque developed by the reaction of the current passing through said coils with the magnetic ileld in which said rigid unit is oscillating,

a gradient along the oscillation axis of said coils.

2. The system of claim l, having means for adjusting the angular position of said coils with regard to each other.

CHARLES H. FAY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,831,744, Laurent Nov. 10, i931 1,863,415 Bieber June 14, 1982 2,954,672 Edgar Sept. l5, 1936 2,123,045 Hoare July 5, 1938 2,151,627 Vacqulcr Mar. 21, 1939 2,220,788 Lohman Nov. 5, 1949 2,379,716 Huil July 3, 1946 2,407,202 Vacquier Sept. 3, 1948 FQREIGN PATENTS Number v Country Date 397,187 Great Britain Aug.14. 1938 

